When a communication path is established between a calling and called party, if one of the parties to the call is associated with a key telephone system, it is possible for the key telephone party to place the call in the hold mode. In this mode the communication path between the parties is maintained active but communications cannot take place. The key telephone party can, by operation of the line key, return the call to the bidirectional communication state.
In some situations it may be desirable to provide some form of entertainment, usually music, to the party placed on hold. In other situations it may be more desirable to provide a pleasing tone or combination of tones to the party placed on hold.
It is desirable, for economic reasons, to use the same line circuit to serve all stations whether or not a tone or music-on-hold feature is provided. Such a system is described in U.S. Pat. No. 4,056,695 issued to Angner et al on Nov. 1, 1977. However, the system described in the above-mentioned circuit while constituting an improvement over prior art requires rewiring of the line card to accommodate the connection of a connectorized daughter-board music receiver card to the line card. In certain applications spare terminals on the line card are not available to wire-in the music signal and hence a separately mounted tone or music coupler card is required thereby resulting in additional space allocation, additional wiring and duplication of hold control logic on the music receiver card. In addition, the existing tone and music coupler cards use passive components which exhibit limited isolation between telephone lines sharing a common music source and could also exhibit signal loading problems when too many lines share a common tone or music source.
In copending parent U.S. patent application for a "Modulated RF Carrier Distributing Arrangement for Key Telephone Line Circuits" Ser. No. 846,144 filed Oct. 27, 1977 which is now U.S. Pat. No. 4,149,042, a modulated RF carrier distributing arrangement is disclosed which will provide a music-on-hold feature for the key telephone unit (KTU) line circuits of the type described by U.S. Pat. No. 4,056,695. The system consists of a centralized music transmitter unit and optional connectorized music receiver cards which are connected to each line card requiring a music-on-hold feature. Signal communication between the common transmitter and the music receiver cards is accomplished using the existing common dc power wiring to each line card. The music transmitter unit frequency modulates an internally generated RF carrier with a customer provided music signal. The frequency modulated (FM) signal is then applied to a common dc power supply lead and distributed to the line cards. The music receiver is constructed as a connectorized daughter-board which mates with the existing option connector on the line card. The existing option connector has dc power and ground voltages available in addition to leads connected across the existing hold impedance. The music receiver obtains dc power as well as the FM signal over the power lead and then demodulates the received signal which is then applied directly across the hold impedance of the line card.
In the pending patent application of Biggs Ser. No. 846,144 filed on July 14, 1977 which is now U.S. Pat. No. 4,090,038, a tone-on-hold circuit is disclosed which utilizes light coupled transistors to couple a common digital signal source to several telephone lines on hold. The Biggs' application while offering excellent isolation between the lines does not teach a self-contained line powered tone circuit which functions as both a line hold impedance and a tone source.
Thus, in the existing art the application of an audio tone or music to a telephone line in the hold state is usually accomplished by adding circuitry across an existing hold impedance, this circuitry requiring connection to the line circuit power supply and a common external tone or music source.